High impact strength blend of isotactic polypropylene, polyethylene and polyisobutylene



United States Patent i s 192 zss HIGH IMPACT sTRENhrrr BLEND orIsorAcrrc roLYPRorYLENE, POLYETHYLENE AND POLY- ISOBUTYLENE Andrew F.Sayko and James P. Forsman, Westfield, N.J.,

assignors to Esso Research and Engineering Company, a corporation ofDelaware No Drawing. Filed Aug. 21, 1961, Ser. No. 132,604

4 Claims. (Cl. 260-897) Inthe recent past, so-called low pressurepolypropylene has been attracting increasing attention as a plasticcompetitive with polyethylene. 'This polymer has been prepared by thenew well known process of polymerizing propylene with catalyst systemsmade up of reducible, heavy, transition metal halides and a reducing,metalcontainingv compound, to high density, isotactic, high molecularweight, solid, relatively linear products. One of the drawbacks of suchlow pressure polypropylene plastics has been their excessive brittlenessat low temperatures. In particular, this has tended to limit theirutility as a packaging material for frozen foods, wire and cableinsulation, plastic pipe and bottles, etc. Low temperature brittlenessis commonly measured by the Bell Brittleness Temperature Test (ASTM TestD-746). Im-

pact resistance (ASTM Test D-256), which is another form of brittlenessmeasurement, represents a similar problem.

It has now been found that the low temperature and impact resistanceproperties of the polypropylene are improved by blending it with rubberyVistanex of controlled molecular weight and small amounts ofpolyethylene of controlled properties. The resulting compositions have aBell brittleness temperature as low as F. and improvements in otherproperties such as impact and tear strengths.

It is surprising that this improvement should be obtained since theVistanex itself does not impart a qualitative improvement evenapproaching that of the blend of the three materials. The small amountsof polyethylene itself also fails to give equivalent improvement inpolypropylene blends. In addition, the use of the three components makesthe processing into the blend compositions much more efiicient andavoids problems of incompatibility.

The low pressure isotactic polypropylene polymers used can havemolecular weights in the range of about 50,000 to 300,000 or even ashigh as 3,000,000 as determined by the intrinsic viscosity method usingthe I. Harris Correlation (I. Polymer Science, 8, 361, 1952). Thepolymers have a high degree of crystallinity and a low solubility inn-heptane. The density (73 F./ g. cc.) is in the range of about 0.88 to0.91 (i.e., molded pads annealed in boiling water for 1 hr.-; densitydetermined by a weight ditference in alcohol and air). Thepolypropylene-is utilized in an amount of from 70 to 90 weight percentbased on the total blend.

Vistanex is employed in an amount of from 9.5 to 28.5 wt. percent basedon the total blending. Generally Vistanex is employed in an amount offrom 9.5 to 25 wt. percent based on the total blending.

The polyethylene employed is the so-called lower density, high pressurepolyethylene, also known as DY LT (see Golding, Polymers and Resins(1959), pages 374 ice to 385). The material used herein has a Staudingermolecular weight of 12,000 to 28,000 and a density of about 0.92(density determined in same manner as for polypropylene). Thepolyethylene is employed in an amount of from 0.5 to 5.0 weight percent,generally from 2.0 to 5.0 weight percent, based on the total blend.

It is significant that the use of Vistanex or polyethylene outside ofthe quantity or molecular'weight limitations indicated herein does notyield equivalent results.

The admixing of the polymers can be done by conventional means such asby milling, extruding, Banbury mixing and other known procedures.Itisparticularly preferred that the polyethylene first be admixed withthe polyisobutylene at a temperature in the range of 250 to' 350 F. fora time interval of l to 5 minutes. The blend of'these two components isthen admixed with the poly In Examples 1 to 3, compositions containingonly polypropylene and Vistanex were made by adding thelatter to themolten polypropylene on the mill. When three polymers were employed, theDYLT was incorporated into the Vistanex and-polypropylene then added tothis mixture on the mill.- By this procedure the Vistanex is plasticizedprior to blending with polypropylene. Milling periods of ten minuteswere maintained for all compounds.

Test specimens consisted of mil compression molded pads and 2 milextruded film. The former were molded atv 400 F. and were used fordetermining tensile strengths and Bell brittleness values. Yieldtensiles were measured at 75 F. on an Instron apparatus at an extensionrate of 2 in./min. Brittleness measurements were made on A x 1%" stripsand the temperature noted when more than 2 of the 4 samples fracturedunder impact.

Films were extruded with the Killion small scale extruder with the dieat 500 F., barrel #1 at 450 F. and barrel #2 at 475 F. In order toequalize orientation effects a constant pull down rate was maintained.Film impact and tear strengths were obtained with the Elemendorf TearingTester.

EXAMPLE 1 Blend 75 F., 40 F. impact, 75 F. 40 F. tear,

p.s.i. jmn, TD

These figures demonstrate the marked improvement in impact and tearstrength (Elmendorf Tearing Tester) obtained through the incorporationof minor amount-s of polyethylene.

EXAMPLE 2 Blends were prepared with polypropylene using area,

materials similar to those in Example 1. The results were as follows:

Blend 75 F.,'40 If. impact, Bell brittleness, F.

These results further demonstrate the marked improvement in impactstrength and low temperature flexibility given by polyethylene addition.

' EXAMPLE 3 A third series of-runs were made with 70% polypropylcne.Theresults were as follows:

Blend 75 11, 40 1i. impact, Bell brittleness, F.

p.S.l.

These results show that as polypropylenecontent decreases, theeffectiveness of the polyethylene also decreases at concentrations aboveEffective blends, therefore, must contain a minimum of 70%polypropylene.

. Other tests demonstratedthat the use of Ll 20 (100,000 1 to 120,000molecular weight), and 15-250 (200,000 to Add L-80 and DYLT'Banbury at330 F 0.

Add A polypropylene to makemixture flux 2 Add remainder polypropylene 8DumpFinal temperature=370 F 14 Test results 1 Banbury Millrnix Bellbrittleness, 9 F .i "1;. 1 0 0 Impact, psi

This example demonstrates that Banbury and mill mixing give equivalentresults.

EXAMPLE" 5 Since one of the main applications for this type of blend isin the heavy duty shipping bag market, the durability of the films ofthe compositions of this invention was.

tested by drop tests. These tests were carried out on bags filled withpounds of dry sand and tied at both ends with cord.i The bags weredroppedrepeatedly from a 4 v height of four feet on to smooth'concrete.A standard order of dropping was used-front, side, rear, side, end, etc.The following table shows the results obtained on polypropylene,Vistanex blend with and without added polyethylene compared with controlsamples.

(1) Three of five specimens were unbroken after 12 drops.

Even at only 4 mils thickness, the ternary blends cold performance isdefinitely better than'that of polypropylene and Vistanex alone.

The advantages. of this invention will be apparent to those skilled inthe art. Improvements in the physical characteristics of thepolypropylene plastics are obtained in an efficient and economicalmanner. Chiefiy improved are the Bell brittleness temperaturecharacteristics,tear strength and impact resistance.

It is to be understood that this invention is not limited to thespecific examples which have been oltered merely as illustrations andthat modifications may be made without departure from the spirit of theinvention.

What is claimed is:

ll. A composition of matter of improved low tempera, ture and impactresistance characteristics comprising a blend of solid, isotacticpolypropylene, polyisobutylene having a molecular weight in the range of60,000 to 80,000 and polyethylene having a molecular Weight in the rangeof 12,000 to 28,000 and a density of 0.92, the polymers beingutilized inthe blend in amounts of respectively to 9.0 weight percent, 9.5 to 28.5weight vpercent, and0.5 to 5.0 weight percent.

2. A process for preparing a polymer blend of improved low temperatureand impact resistance characteristics, said blend containing from 70 toweight percent solid iso tactic polypropylene, from 9.5 to 28.5 weightpercent polyisooutylene having a molecular weight in the range of 60,000to 80,000, and from 0.5 to 5.0 weight percent polyethylene having amolecular weight in the range of 12,000 to 28,000 and a density of 0.92which comprises admixing the polyethylene and polyisobutylene at atemperature in the range of from 250 to 350 F. and then admixing thismixture with the polypropylene at a temperature in the range of from 340to 380 F.

3. The process of claim 2 in which the polypropylene isadded to themixture of the other materials-in a stepwise manner.

4. A composition of'matter comprising a blend of 70 to 90 percent byweight of crystalline polypropylene, 2 to 5 percent by weight ofpolyethylene and 9.5 to 25 percent by weight of rubbery polyisobutylene.

References Cited by the Examiner UNITED STATES PATENTS 2,854,435 I 9/58Briggs et a1. 260-897 2,956,042 10/60 Underwood at al 260-897 2,993,0287 7/61 Ranalli 260897 MURRAY TILLMAN, Primary Examiner. WILLIAM H.snonr, LEON J. eancovrrz,

Examiners.

4. A COMPOSITION OF MATTER COMPRISING A BLEND OF 70 TO 90 PERCENT BY WEIGHT OF CRYSTALLINE POLYPROPYLENE, 2 TO 5 PERCENT BY WEIGHT OF POLYETHYLENE AND 9.5 TO 25 PERCENT BY WEIGHT OF RUBBERY POLYISOBUTYLENE. 